Small mammals often huddle together in groups. The reasons for huddling behaviour may be diverse. However, theoretical models based on the geometry of exposed surface areas (e.g. Canals, Rosenmann & Bozinovic 1989, 1997), and direct measurements of the energy expended by animals grouped together in huddles (Andrews & Belknap 1986; Bryant & Hails 1975), suggest that an inevitable consequence of huddling may be that the energy expenditures of the individual animals in the huddle are reduced when compared with solitary individuals. This energy saving may itself be a selective advantage favouring the evolution of huddling behaviour in certain circumstances (Sealander 1952; Fedyk 1971; Gebczynska & Gebczynski 1971; Baudinette 1972; Karasov 1983; Springer, Gregory & Barrett 1981).
The physical basis for the reduction in energy expenditure of huddled animals has been the focus of some debate. On one hand the effect may be due to reductions in the mean individual exposed surface area of huddled vs solitary animals (Contreras 1984; Vickery & Millar 1984; Canals et al. 1989). Alternatively huddles of animals may elevate the local temperatures more effectively than solitary animals, meaning the local temperature experienced by the huddle is greater than that experienced by solitary individuals (Andrews, Phillips & Makihara 1987; Hayes, Speakman & Racey 1992a). Hayes et al. (1992a) partitioned these effects, controlling for differences in activity between huddled and solitary Short-Tailed Field Voles (Microtus agrestis), and found that approximately half the observed saving could be attributed to each effect. Canals et al. (1997) suggested the majority of the effect was accounted for by reduced surface area.
Apart from the physical effects of huddling on energy expenditure there may also be direct biological effects of animals upon each other when they are in close proximity. Thus two animals huddling together may have a calming effect on each other and mutually reduce each other’s metabolic rates. For example, Martin, Fiorentini & Connors (1980) found that single white mice (Mus sp.) and gerbils (Meriones unguiculatus) had greater metabolic rates than huddled trios in the same chamber, or of the same three animals separated by partitions to prevent huddling. In direct contrast to these findings, however, Contreras (1984) found no suppressive effect on metabolism when trios of mice and gerbils were measured together but separated by partitions, indicating no social effect on metabolic rate.
In part the different results found by Martin et al. (1980) and Contreras (1984) may reflect subtle differences in experimental design and the fact that in some circumstances there may be more direct physiological effects of one animal on the metabolism of its immediate neighbours. Herreid & Schlenker (1980) found that if a mouse was placed in a completely separate chamber upstream of a second mouse, the metabolism of the second mouse was suppressed for the duration that the two chambers were connected. These experiments led them to suggest that an airborne factor was responsible for suppressing the metabolism of the second mouse.
In a second paper, Schlenker & Herreid (1981) performed some further manipulations using the same system to try to establish the nature of the airborne factor. The conclusion of this work was that the suppressing factor was elevated levels (0·2–0·8%) of CO2 in the second chamber owing to exhaled CO2 from the first mouse. In further experiments, Schlenker, Carlson & Herreid (1981) demonstrated that the effect was still present in anosmic mice, indicating that the effect was mediated by CO2 and not some other olfactory cue.
Observations of socio-physiological effects of small mammals on each other have several potential implications for observations of energy savings when animals come together in huddles. In particular, at least part of the observed saving may be due to social and physiological effects rather than any physical effect due to reduced surface area or local heating. Whether animal metabolic rates are reduced simply by the presence of conspecifics, or whether suppression is additionally mediated physiologically by low ambient levels of CO2 is an important question that has ramifications not only for our understanding of why energy savings might occur in huddled animals, but also for respirometry measurements of energy expenditure in general. Measurements of resting and basal metabolic rates, for instance, are always made for animals that are solitary, and, if an open flow system is employed, also in the CO2-rich atmosphere of a respirometry chamber. If there is a ‘calming’ effect when other individuals are present that is absent if the animal is alone, this may have ramifications for the interpretation of the practical significance of BMR and RMR measurements. In particular, are solitary animals in the wild in the ‘calm’ state, or in the same state as that pertaining in measurement protocols for BMR and RMR? Alternatively, does the artificially high level of CO2 in a respirometry chamber lead to suppressed metabolism relative to that normally experienced by animals in the wild? These potential effects may contribute to the reported discrepancies between time and energy budget calculations of daily energy expenditures of animals, and more direct measurements based on the doubly labelled water technique (Weathers et al. 1984; Nagy 1989). In the current paper we report a series of experiments in which we aimed to assess the effects of social and physiological suppression on the metabolism of white mice.